Long Term Evolution (LTE) Advanced is a mobile telecommunication standard proposed by the 3rd Generation Partnership Project (3GPP) and first standardised in 3GPP Release 10. In order to provide the peak bandwidth requirements of a 4th Generation system as defined by the International Telecommunication Union Radiocommunication (ITU-R) Sector, while maintaining compatibility with legacy mobile communication equipment, LIE Advanced proposes the aggregation of multiple carrier signals in order to provide a higher aggregate bandwidth than would be available if transmitting via a single carrier signal. This technique of Carrier Aggregation (CA) requires the original data to be split into multiple data streams, which are modulated separately onto a number of carrier signals. Each of the signals are then demodulated at the receiver, whereafter the message data from each of the signals can be combined in order to reconstruct the original data. Carrier Aggregation can be used also in other radio communication protocols such as High Speed packet Access (HSPA).
FIG. 1 illustrates two main variations of CA on frequency-amplitude graph 100. The graph areas 102, 104, 106 and 108 are representative of signals modulated at carrier frequencies fC1, fC2, fC3 and fC4 respectively. Carrier frequencies fC1, fC2, fC3 and fC4 are associated with a set of adjacent channels used in a communication scheme, such as LTE, which has been assigned for use in the displayed section of the frequency spectrum. Each communication channel may be separated by guard bands, which are unused sections of the frequency spectrum designed to improve the ease with which individual signals can be selected by filters at the receiver and reduce the likelihood of interference between signals transmitted in adjacent channels.
In a first communication instance, data is transmitted using the aggregation of signals 102 and 104 modulated at carrier frequencies fC1 and fC2 respectively. This is an example of contiguous CA, where data is transmitted at carrier frequencies that are adjacent in the frequency spectrum. In a second communication instance, data is transmitted using the aggregation of signals 106 and 108 modulated at carrier frequencies fC3 and fC4 respectively. This is an example of non-contiguous CA, where data is transmitted at carrier frequencies that are separated by one or more intermediate carrier frequencies (in this case fC1 and fC2) not used in the communication instance. In some non-contiguous CA arrangements, the aggregated signals may be in entirely different frequency bands.
Several radio communication schemes, including LTE, use quadrature modulation to increase the data density of a single frequency channel by transmitting a second message which is modulated with a carrier that is 90° out of phase with respect to a first message. These two message components are termed the in-phase (I) and quadrature (Q) components respectively. A common method for generating a quadrature modulated signal uses a transmitter arrangement known as a Direct Conversion Transmitter (DCT).
FIG. 2 illustrates a schematic diagram of an exemplary DCT as known in the art. The original data is first split into two message streams, which are used to generate the desired I and Q signals 202 and 204 in the digital domain. Digital to analogue converters (DACs) 206 and 208 convert the binary representations of the I and Q message data into baseband I and Q signals. The desired I and Q components are isolated using low pass filters 210 and 212 respectively, which are used to suppress unwanted frequencies, such as harmonics or artefacts associated with the conversion process generated outside of the intended frequency range.
In order to achieve the required frequency up-conversion, mixers 214 and 216 perform multiplication between the input signal and a locally generated signal of the required carrier frequency, generated by local oscillator 218. In order to uniquely modulate both the I and Q components, the baseband signal must be mixed with both in-phase and quadrature shifted versions of the local oscillator signal, which are generated by quadrature generator 220. The I component of the signal is mixed with the in-phase local oscillator signal by mixer 214, and the Q component of the signal is mixed with the quadrature phase local oscillator signal by mixer 216.
The two up-converted signals are then summed at radio frequency at summing point 222. The summed signal is them amplified by power amplifier 224 to a level suitable for transmission. As antenna 228 is typically shared with other hardware, such as corresponding receiver circuitry, a band pass filter 226 (commonly referred to as the duplex filter) is typically deployed. This prevents signal power from the transmitted signals from leaking into the antenna at frequencies to which the receiver is sensitive. Finally, the combined radio frequency signal is emitted via antenna 228.
With minimal modification, a DCT can also be used to transmit a contiguous CA signal, i.e. one which involves the aggregation of two adjacent frequency channels. Signals 102 and 104 of FIG. 1 are an example of CA according to such a configuration. One type of contiguous CA scheme is defined in 3GPP TR 36.807 as Carrier Aggregation bandwidth class C.
FIG. 3 illustrates the operation of conventional DCT hardware when used to transmit data via two adjacent signals on frequency-amplitude graph 300. Signals 302 and 304 are synthesised in the digital domain centred on frequencies at plus and minus half the required channel separation distance (fSEP) of the CA signals. Here and subsequently, the negative portion of the frequency axis is used to indicate a 180° phase shift, as is common convention. Mixing a signal having frequency f with a local oscillator signal of frequency fLO has the effect of translating the signal to be centred on new frequencies at fLO−f and fLO+f. However, careful construction of the signals in the digital domain allows controlled up-conversion to only one of these frequencies. The following description utilises the convention where the negative frequency signal is translated to the lower frequency carrier and the positive frequency signal is translated to the higher frequency carrier. An alternative convention is to invert this relationship.
After digital to analogue conversion, mixing the synthesised signals 302 and 304 with local oscillator signal 306 having a frequency halfway between the required carrier frequencies (fC1 and fC2), results in frequency up-conversion of signals 302 and 304. Signal 302 is translated to be centred on frequency fLO−½fSEP (i.e. fC1) as shown by translated signal 308 and arrow 312. Signal 304 is translated to be centred on frequency fLO+½fSEP (i.e. fC2) as shown by translated signal 310 and arrow 314. Under this arrangement the DCT hardware acts as a low intermediate frequency (IF) transmitter for both carriers.
Unfortunately, due to imperfections in the transmitter hardware (such as component mismatch, filter quality, quadrature signal phase quality etc.), the I and Q branches of a real transmitter will have finite amplitude and phase balance. As a result, some of the carrier power of each signal leaks into the other side. This is conventionally, conceptually represented as an image signal “folded” around the local oscillator frequency.
FIG. 4 shows the effect of these imperfections on the transmitted signals on frequency-amplitude graph 400. Again, signals 402 and 404 are frequency up-converted by mixing them with local oscillator signal 406. Due to the hardware imperfections discussed previously, the up-conversion process not only results in a translation of each signal to fLO+f; more particularly, it also generates image signals at fLO−f which are folded around the local oscillator frequency and have a smaller but proportional magnitude. Mixing signal 402 with local oscillator signal 406 therefore results in a translation to signal 408 as shown by arrow 412, as well as the generation of image signal 416 as shown by arrow 420. Likewise, mixing signal 404 with local oscillator signal 406 results in a translation to signal 410 as shown by arrow 414, as well as the generation of image signal 418, as shown by arrow 422.
Hence, the up-conversion process results in the generation of an image signal over each of the carrier signals, with a magnitude that is proportional to that of the other carrier signal. Functioning DCTs are designed with a finite Image Reject Ratio (IRR) that is sufficient to reject images of this proportional magnitude. However, problems arise when the required signal strengths of the two carrier signals are substantially different. This could occur for several reasons. For example, due to the different propagation characteristics of the different frequency carrier signals, one carrier signal may become more attenuated on route to the intended recipient, and hence require transmission with greater signal strength. Additionally, one of the carrier signals could be augmented through the provisioning of frequency selective repeaters, thereby lowering the required signal strength of one signal relative to the other. Further, different carrier frequencies may be associated with different cell coverage areas or different transmitter directivity.
Each path between the transmitter and an intended recipient will have an associated propagation delay, which describes the amount of time required for a signal to travel along the path. In order to properly schedule the receipt of the carrier signal at the recipient, a time advance parameter is commonly used to determine when to begin the transmission of a given carrier signal. When transmitting more than one carrier signal, a multiple time advance parameter can be employed in order to configure the time advance of each carrier signal independently. As a result, the two carrier signals may be transmitted with an offset in the time domain. The use of a multiple time advance parameter thus enables the transmission of carrier signals which follow different propagation paths to their intended recipient, including the transmission of carrier signals to different recipients entirely. When the two carrier signals have different propagation paths to their intended recipient, it is likely that the propagation paths will have different associated attenuation characteristics. In order to enable successfully transmit the tow carrier signals under such conditions, the two signals may be required to be transmitted with different signal strengths.
FIG. 5 illustrates the operation of conventional DCT hardware when used to transmit data via two adjacent signals requiring a relative signal strength imbalance on frequency amplitude graph 500. In this scenario, the signal strength required for signal 508 (i.e. the carrier signal at fC1) is significantly lower than the signal strength required for signal 510 (i.e. the carrier signal at fC2). This is achieved by generating signal 502 with a proportionally smaller amplitude than signal 504. Again, signals 502 and 504 are frequency up-converted by mixing them with local oscillator signal 506. Mixing signal 502 with local oscillator signal 506 results in a translation to signal 508 as shown by arrow 512, as well as the generation of image signal 516 as shown by arrow 520. Likewise, mixing signal 404 with local oscillator signal 406 results in a translation to signal 410 as shown by arrow 414, as well as the generation of image signal 418 as shown by arrow 422. Unlike the previous example, where there was no signal strength imbalance between the transmitted carriers, there is now a large image (generated by the more powerful carrier signal) over the less powerful carrier signal, and a small image (generated by the less powerful carrier signal) over the more powerful carrier signal.
In the case of the carrier signal at fC1, the overlapping image signal now has a much larger proportional magnitude which the finite IRR of the transmitter may not be capable of suppressing. If this is the case then the image signal will significantly deteriorate the constellation and error-vector magnitude of the weaker carrier. As the required signal strength imbalance increases, the greater this effect becomes, and hence higher I-Q performance/IRR is required for reliable operation. Hence there is a finite limit to the signal strength imbalance between two contiguous CA carriers that can be successfully transmitted by a single conventional DCT transmitter path before the weaker carrier becomes too degraded for reliable transmission, or the necessary quality hardware components become prohibitively expensive.
FIG. 6 schematically illustrates an alternative known hardware arrangement for transmitting data via two adjacent signals requiring a relative signal strength imbalance. In FIG. 6, a dedicated transmitter path is provided for the generation of each of the two CA signals. A first transmitter path contains DACs 606 and 608; low-pass filters 610 and 612; mixers 614 and 616; local oscillator 618; quadrature generator 620; summing point 622, power amplifier 624; duplex filter 626; and antenna 628. A second transmitter path contains DACs 636 and 638; low-pass filters 640 and 642; mixers 644 and 646; local oscillator 648; quadrature generator 650; summing point 652, power amplifier 624; duplex filter 626; and antenna 628. The operation of DACs 606, 608, 636 and 638; low-pass filters 610, 612, 640 and 642; mixers 614, 616, 644 and 646; quadrature generators 620 and 650; summing points 622 and 652; power amplifier 624; duplex filter 626; and antenna 628; are the same as described previously in relation to FIG. 2. However, local oscillators 618 and 648 are configured to produce different frequencies by operating at the carrier frequency of the signal intended to be generated by their respective paths. In this manner, the two transmitter paths operate as two individual DCTs, one arranged to generate each carrier signal.
FIGS. 7a and 7b illustrate the operation of the alternative known hardware arrangement for transmitting data via two adjacent signals having a relative signal strength imbalance. The operation of the first transmitter path is shown on frequency amplitude graph 700a. Signal 702 is generated at baseband and mixed with local oscillator signal 706a operating at one of the required carrier frequencies (fC1). This results in the translation of signal 702 to fC1, as shown by translated signal 708 and arrow 712, as well as the generation of image signal 716, folded around the local oscillator frequency. As the local oscillator frequency is the same as the carrier frequency in this case, image signal 716 is generated on top of carrier signal 708, as shown by arrow 720.
The operation of the second transmitter path is shown on frequency amplitude graph 700b. Signal 704 is generated at baseband and mixed with local oscillator signal 706b operating at one of the required carrier frequencies (fC2). This results in the translation of the signal to fC2, as shown by translated signal 710 and arrow 714, as well as the generation of image signal 718, folded around the local oscillator frequency. Again, as the local oscillator frequency is the same as the carrier frequency, image signal 718 is generated on top of carrier signal 710, as shown by arrow 722.
Since the magnitude of image signals 716 and 718 are now proportional to signals which they overlap (708 and 710 respectively), the finite IRR of the transmitter can be expected to suppress these images effectively. The two generated signals can then be summed prior to transmission to form the required power imbalanced CA signals.
Due to the use of independently configurable local oscillators, this method is more commonly used for noncontiguous carrier aggregation, where the two signals may be transmitted at very different carrier frequencies and the single DCT operating as a low-IF transmitter for both carriers (as described previously) is not appropriate. However, when applied to a contiguous carrier aggregation configuration, the two local oscillators operate at very similar frequencies. In single integrated circuit deployments, the configuration may suffer from local oscillator pulling due to difficulties in sufficiently isolating the two local oscillators from one another. The effect of this is to cause instabilities in the generated signals as the two operating frequencies tend towards each other, thereby impeding the successful operation of the transmitter. Additionally, the complexity of the summing point and associated losses are increased. Further, this arrangement has increased silicon area and power consumption costs when compared to the single. DCT arrangement described with reference to FIG. 2, which make it a less desirable solution. Hence, it is an objective of the present disclosure to provide improved transmitter hardware, capable of effectively transmitting data via aggregated carrier signals.